1. Field of the Invention
The present invention relates to a reflected power suppression circuit. More particularly, the present invention relates to a circuit for preventing power reflected from an antenna from being inputted to a power amplifier.
2. Description of the Background Art
In a portable terminal of a mobile communication device such as a mobile phone and a PHS (Personal Handyphone System), due to its portability and reduced size, an antenna often gets damaged or contact with metal such as a steel plate of a steel desk. With the antenna used in such a condition, if the portable terminal originates a call or changes its state to a communicating state, and radiates a transmission power, the transmission power is reflected from around the antenna, whereby the matching condition between a power amplifier and the antenna is no more satisfied. When the output matching condition of the power amplifier is not satisfied due to the reflected wave power, a load curve of the power amplifier changes, and the power amplifier operates outside of the Area of Safe Operation (ASO) which is set at the design process. As a result, there is a possibility that the power amplifier may be broken or damaged. Thus, an amplifying device structuring the power amplifier used in such a portable terminal is required to have a high degree of immunity against a load change.
In recent years, GaAs-HBT (Hetero junction Bipolar Transistor) has been developed and put into practical use as a power amplifier of the portable terminal for improving device efficiency. However, the GaAs-HBT has a lower degree of immunity against a load change compared to conventional devices such as Si-MOSFET and GaAs-MESFET. As a result, in the case where the antenna is broken, for example, in the portable terminal with the power amplifier to which the GaAs-HBT is applied, there is a high possibility that the power amplifier becomes broken or damaged. Thus, in order to put the GaAs-HBT, which is a high-quality and high-efficiency device, into practical use as a power amplifier of the portable terminal, the GaAs-HBT should be prevented from being damaged by a load change.
As a method for solving the above problem, there exists a portable terminal provided with an isolator inserted between a power amplifier and an antenna. Also, there exists a portable terminal provided with a directional coupler inserted between a power amplifier and an antenna for suppressing a transmission power of the power amplifier based on reflected power outputted from the directional coupler. Hereinafter, with reference to the drawings, these portable terminals will be described. FIG. 8 is a block diagram showing a configuration of a portable terminal provided with the isolator. Also, FIG. 9 is a block diagram showing a configuration of a portable terminal provided with the directional coupler.
First, the portable terminal as shown in FIG. 8 will be described. The portable terminal as shown in FIG. 8 includes an input terminal 101, an output terminal 102, a power amplifier 103, a power supply terminal 104, and an isolator 105. A transmission signal is inputted from the input terminal 101. The power amplifier 103 amplifies the transmission signal, and outputs the amplified signal. The power supply terminal 104 is a terminal through which power for driving the power amplifier 103 is supplied. The isolator 105 outputs the transmission signal outputted from the power amplifier 103 to the output terminal 102, and outputs a signal reflected from the output terminal 102 to the power amplifier 103 to a ground. The output terminal 102 outputs the transmission signal outputted from the isolator 105 to the antenna.
In the portable terminal configured as described above, the power amplifier 103 is prevented from being damaged due to breakage, etc., of the antenna. Detailed descriptions are provided below.
In the portable terminal, breakage, etc., of the antenna increases the power reflected from the antenna to the power amplifier 103. Due to such reflected power, the matching condition between the power amplifier 103 and the antenna may not be satisfied. When the matching condition between the power amplifier 103 and the antenna is not satisfied, a load curve of the power amplifier 103 changes, and the power amplifier 103 operates outside of the ASO which is set at the design process. As a result, the power amplifier 103 becomes damaged.
The power amplifier 103 as shown in FIG. 8 is provided with the isolator 105 inserted between the power amplifier 103 and the output terminal 102. The isolator 105 outputs the reflected power to the ground, whereby the reflected power is not inputted to the power amplifier 103, even if the reflected power is increased due to breakage, etc., of the antenna. Accordingly, the matching condition between the power amplifier 103 and the antenna remains satisfied, and the load curve of the power amplifier 103 remains unchanged. As a result, the power amplifier 103 always operates within the ASO which is set at the design process, and damage to the power amplifier 103 is prevented.
Next, the portable terminal as shown in FIG. 9 will be described. The portable terminal as shown in FIG. 9 includes the input terminal 101, the output terminal 102, the power amplifier 103, the power supply terminal 104, a directional coupler 106, a detection circuit 107, a control circuit 108, and a termination resistor 109. A transmission signal is inputted from the input terminal 101. The power amplifier 103 amplifies the transmission signal, and outputs the amplified signal. The power supply terminal 104 is a terminal through which power for driving the power amplifier 103 is supplied. The directional coupler 106 outputs the transmission signal outputted from the power amplifier 103 to the output terminal 102, and also outputs, to the detection circuit 107, the power reflected from the output terminal 102 to the power amplifier 103. The detection circuit 107 converts the reflected power to a voltage signal, and outputs the voltage signal to the control circuit 108. The control circuit 108 controls the power gain of the power amplifier 103 based on the intensity of the voltage signal.
In the portable terminal configured as described above, damage to the power amplifier 103 caused by a broken antenna, etc., is prevented. Detailed descriptions are provided below.
In the portable terminal, power reflected from the antenna to the power amplifier 103 is increased due to breakage, etc., of the antenna. When reflected power is increased in such a manner, the power amplifier 103 is damaged.
Thus, in the portable terminal as shown in FIG. 9, the directional coupler 106 detects the reflected power, and the detection circuit 107 converts the reflected power to a voltage signal. Furthermore, when the voltage signal is high, the control circuit 108 reduces the power gain of the power amplifier 103. As such, by controlling the power gain of the power amplifier 103, the load curve is controlled so as to be within the Area of Safe Operation, even if the matching condition between the antenna and the power amplifier 103 is not satisfied due to the reflected power. As a result, damage to the power amplifier 103 is prevented.
However, the portable terminal as shown in FIG. 8 has drawbacks in that a size of the terminal cannot be reduced and a power loss occurs in the direction of transmission. Detailed descriptions are provided below.
The isolator is a magnetic circuit. Thus, the size of the isolator cannot be reduced unless the relative permeability of materials of the magnetic circuit is dramatically increased. A commercialized isolator operating at a frequency range of 800 MHz to 2 GHz is generally used in the portable terminal, and a size thereof is a square 5–7 mm on a side. This is equal to or greater than the size of the power amplifier. Thus, the isolator is a great obstacle preventing the portable terminal from being reduced in size. Also, the isolator has constant forward direction transition loss characteristics regardless of the presence or absence of the reflected power. Thus, in the portable terminal, power loss always occurs when a signal is transmitted.
Also, the portable terminal as shown in FIG. 9 has a drawback in that a power loss occurs in the direction of transmission. Specifically, since the directional coupler 106 has a constant degree of coupling, the directional coupler 106 has constant forward direction transition loss characteristics regardless of the presence or absence of the reflected power. As a result, in the portable terminal, power loss always occurs when a signal is transmitted.
Note that the method for preventing the power amplifier from being damaged is also disclosed in Japanese Laid-Open Patent Publication Nos. 6-268536, 2002-76791, and 2000-165164.